Sidisi’s last ability triggers any time creature cards are put into your graveyard from anywhere in your library, not just from the top.

Any instruction, including Sidisi’s first ability, that tells you to put multiple cards from a library into a graveyard moves all the cards at the same time. For example, if Sidisi enters the battlefield and you put three creature cards into your graveyard, Sidisi’s last ability will trigger once and you’ll put one Zombie token onto the battlefield.